Photometry

Arguably one of the more challenging aspects of amateur astronomy is to electronically record consistently accurate stellar magnitudes. Don’t let this statement deter you. Once the technique is mastered your precious data is highly sought after by the AAVSO and other repositories.

The requirements for photometry are not trivial. A telescope, preferably an all reflecting Classical Cassegrain but any good scope will suffice, a steady and accurate equatorial mount, a good mono CCD ideally without anti blooming gates, a set of UVBI Johnson – Cousins photometric filters or at least the V filter, a computer with appropriate software and a lot of dedication. If you have a DSLR and a good lens your domain is differential photometry.

Photometry is not just photometry. Photometry  using a mono CCD  has its set of rules and requirements, DSLR photometry requires a completely different approach to achieve accurate results and finally the use of photoelectric photometers, known as PEP, has its unique requirements. Irrespective of which instrument you apply and given that all things being equal the results obtained  from each  should be within a magnitude range of .01.

Software

There are a number of software packages that are designed to do  photometry. Listed are a few of the more popular ones. Included are some acquisition software packages for DSLR cameras. Not all the software is free.

  • VPhot – AAVSO – Online photometric tool
  • IRIS – Christian Buil  – DSLR aquisition software – Aperture photometry – Free
  • APT – Aperture Photometry Tool  – Free
  • AIP4Win – Richard Berry, James Burnell – Willman-Bell
  • CCDSoft5 – Software Bisque  – Obsolete
  • The SKY X Pro – Software Bisque
  • IRAF – image processing and  photometry tools – Free
  • MaximDL – Aquisition software – image processing
  • Backyard EOS – Aquisition software for DSLR’s
  • Canopus – SBig – Astrometric & Photometric Software

Organisations to join

These bodies have a vast amount of information including excellent tutorials on how to conduct effective photometry.

Recommended reading

An enormous amount of literature is freely available on the net. For starters read the two documents listed below.

CCD PHOTOMETRY E. Norman Walker
http://www.britastro.org/vss/ccd_photometry.htm
(A very comprehensive page on the subject.)

DSLR Photometry
Photometry 2007SASS___26___67H – John E. Hoot
http://adsabs.harvard.edu/abs/2007SASS…26…67H
(An in depth analysis of DSLR sensors)

Books

The number and titles available on the subject of photometry are too many to mention here. All deal with photometry at quite a high level but are essential to getting a deeper understanding of the subject.

  • Astronomical Photometry / for the Advanced Amateur and Professional Astronomer- Henden and Kaitchuck – Willman-Bell (Recommended by the AAVSO)
  • Introduction to Astronomical Photometry – Edwin Budding, Osman Demircan – Cambridge Online Books (Observing handbook for research astronomers)

Observing Programs

Your skill at making accurate photometric measurements puts you in a good position to detect large transiting exosolar planets or the flux of asteroids and comets.

Where to from here

If you are interested in the field of photometry please contact us and we will assist you in any way we can.

Professional astronomers test results are positive for DSLR Photometry

“High-speed multicolour photometry with CMOS cameras” by SM Pokhvala, BE Zhilyaev & VM Reshetnyk. Advances in Astronomy and Space Physics, Vol. 2, p. 200-202 (download the PDF)

Abstract: We present the results of testing the commercial digital camera Nikon D90 with a CMOS sensor for high-speed photometry with a small telescope Celestron 11” at the Peak Terskol Observatory. CMOS sensor allows to perform photometry in 3 filters simultaneously that gives a great advantage compared with monochrome CCD detectors. The Bayer BGR colour system of CMOS sensors is close to the Johnson BVR system. The results of testing show that one can carry out photometric measurements with CMOS cameras for stars with the V-magnitude up to ≃14^{m} with the precision of 0.01^{m}. Stars with the V-magnitude up to ˜10 can be shot at 24 frames per second in the video mode.